The present invention relates generally to dry etch gas mixtures and plasmas, and more particularly to a dry etch gas mixture and plasma for selectively etching a nitride insulator layer relative to an oxide insulator layer.
In semiconductor chip processing technology, a common insulation structure comprises a layer of a nitride disposed in adjacency with a layer of oxide. There are many processing sequences that require the etching out of the nitride at a particular location with the etch leaving an even and essentially full layer of oxide therebelow. This nitride selectivity over oxide requirement is especially critical for processing contact openings during the formation of transistors.
For example, the FIGURE illustrates a structure that may be used in the formation of a transistor. The structure comprises a silicon bulk area 10, a first layer 12 of SiO.sub.2 disposed over the silicon bulk area 10 and including a reduced thickness area or step 14. The structure further comprises a conformal second layer 16 of Si.sub.3 N.sub.4 disposed over the SiO.sub.2 layer 12, and a conformal thick layer 18 of some form of insulator, such as glass, disposed over layer 16. At the point in the process shown in the FIGURE, an undiffused base dopant 20 is present in a selected area of the silicon bulk below the step area 14 of the SiO.sub.2 layer 12. It is essential that the dopant layer 20 not be penetrated at this point in the process. However, the glass insulator 18 and the Si.sub.3 N.sub.4 in the step area 14 must be removed down to the SiO.sub.2 layer 12 in order to facilitate further process steps. The first step in this etching removal process is to remove the conformal glass insulation layer 18 in the region above the step 14. But, due to the uneven topography of the glass insulation 18 over the step area 14, a predetermined overetch of the glass 18 into the Si.sub.3 N.sub.4 layer 16 is required to remove all of the glass 18 above the step area 14. This overetch results in an uneven Si.sub.3 N.sub.4 topography. Such topography requires a highly selective etch mixture which will etch through the Si.sub.3 N.sub.4, but will stop at the SiO.sub.2 layer 12. Such an etch gas mixture must also have etch homogeneity and good endpoint control.
Most current recipes for the selective etching of a nitride layer in preference to an oxide layer utilize a fluoro carbon gas, either in the plasma mode (high pressure parallel plates or a barrel etcher) or in an afterglow discharge mode via microwave excitation. A typical mixture might comprise CF.sub.4 in 50-95% oxygen. The problem with these etch processes are (1) the etches are isotropic; (2) Polymer formation on the Si.sub.3 N.sub.4 surfaces from the previous oxide etch step interferes with the etch uniformity; and (3) Si.sub.3 N.sub.4 over-etching is limited because of the low Si.sub.3 N.sub.4 -SiO.sub.2 ERR (usually less than 2:1).
The invention as claimed provides a gas mixture and plasma which achieves a selectivity with anisotropic etching of on the order of five to one or greater. Additionally, an etch uniformity for the nitride layer of better than 3% is achieved and the process has excellent repeatability.